1. Field of the Invention
This invention relates to integrated circuit manufacture and more particularly to multi-layered contact structures using argon implantation to control growth of silicide over an ultra-shallow junction.
2. Description of the Relevant Art
An integrated circuit is by definition a number of electrically interconnected circuit elements defined on the same substrate or "chip" Some of the interconnections are done in the silicon substrate itself, but most are done my means of thin conductive strips running across the top surface of the substrate. Each strip is often connected within a contact area to underlying semiconductor materials (often referred to as "junctions"). Contact to junctions must be of low resistivity and is generally as low as a few micro ohms per square centimeter of contact area.
The conductive strips are usually made of aluminum or aluminum alloy, and, in some instances, can have silicon placed therein. Aluminum adheres well to silicon dioxide and has low contact resistance, but may suffer numerous problems, such as, for example, a propensity to grow "spikes".
In order to reduce contact resistance at the interconnect/silicon juncture, many manufacturers utilize a silicide formed at the juncture. The silicide helps break through the residual surface oxide so that good electrical contact can be made. Applying heat necessary for silicidation is sometimes required to adjust the silicon dioxide--silicon interface states. Silicides are made by depositing a thin layer of metal over the entire wafer, heating the wafer to a high enough temperature for the silicon and metal to react in the contact window areas and then etching away the unreacted metal on top of the oxide. Most metals used to form silicide are transition or refractory metals in group IV(B), V(B) and VI(B).
It important that the silicide be grown to a controlled thickness. If the silicide film becomes too thick, defects can occur at the edge of the silicide film due to stresses in the film. Such defects are reported to begin occurring once the thickness of the silicide film exceed approximately 100 nm. The mechanism for growing silicide is generally understood as species of silicon diffusing from the underlying substrate surface to the overlying (and abutting) titanium. If an excessive amount of silicon atoms are allowed to diffuse, then the silicide is made too thick causing undue stresses in the film. It is therefore important to prevent excessive silicon consumption during the growth process. It is important that the integrity of the underlying junction dopant atoms, once placed, remain in their position to maintain the ultra-shallow junction region and the advantages thereof. Specifically, diffusion of boron atoms from the junction into the growing silicide can deplete the dopant within the silicon surface thereby increasing the sheet resistance at the contact area.
The above problems generally present themselves whenever titanium is used as the base silicide metal, and the underlying junction is p-type, highly mobile boron atoms. While the use of titanium presents many problems, there remains many advantages of titanium. Titanium is a mainstay and essential component in modern contact structures. The juncture between titanium and underlying silicon has low sheet resistivity if a silicide region is formed. Yet, the silicide must not be too thick. An optimal silicide thickness is one that is less than 100nm but is thick enough to maintain uniform thickness in all regions across the upper surface topography.